Pulp digesting method and system



Feb. 19, 1935. I F. w. ADAMS ET AL 1,991 5 PULP DIGES'IINGr METHOD AND SYSTEM Filed June 20, 1953 3 Sheets-Sheet 1 zyyz it INVENTORS.

W B) 5 v TTORNEH Feb. 19, 1935. F.'w. ADAMS ET AL 1,991,365

PULP DIGESTING METHOD AND SYSTEM Filed June 20. 1935 a Sheets-Sheet 2 TORIV Feb. 19, 1935'. F. w. ADA MS ET Al. 6

PULP DIGESTING METHOD AND SYSTEM Filed June 20, 1933 {sheets-sheet s Patented Feb. 19, 1935 UNITED STATES PULP DIGESTING LIETHOD AND SYSTEM Frederick W. Adams,

Stoneham, Mass, and

George S. Witham, Jr., Lincoln, N. H.

Application June 20, 1933, Serial No. 676,674

9 Claims.

This invention relates to methods of digesting or cooking pulp and to the apparatus used in such methods. It is more especially, but not exclusively, concerned with the sulphite process of cooking pulp.

As commonly practiced, this process involves the cooking of a charge of wood chips in a large brick lined digester with an acid composed of a mixture of calcium bisulphite, magnesium bisulphite and free sulphur dioxide in solution. After the digester has been filled with chips and acid,

- steam is admitted to it for the purpose of heating.

up the entire charge sufficiently to promote the reaction of the acid on the wood, this reaction consisting chiefly in dissolving or otherwise attacking the lignin which binds the fibers of the wood together, and thus releasing these fibers.

The digesting process may be considered as divided roughly into four steps or stages: first,

heating the charge; second, penetration of the acid into the chips; third, cooking; and finally, recovery. :In a digester having a capacity of, say, ten tons, the heating period may last in a typical cook in the neighborhood of two or three hours, during which time steam is supplied to the charge at a very rapid rate, the digester being vented in the usual manner. By the end of this period the pressure in the digester may have risen to in the neighborhood of seventy-five or eighty pounds per square inch, and thereafter the rate of steam inflow is so reduced as to maintain a substantially constant pressure in the digester. At the end of the heating stage the temperature in the digester usually is between 90 and 120 C. and it is held in this general neighborhood while the acid penetrates the wood and the cooking starts. Usually the temperature during the cooking stage is increased slowly so that some steam must be supplied to the digester continuously during this entire operation. At the end of this stage the temperature may be from 120 to 150 C. The recovery stage coincides with the latter part of the'cooking period when gas is being liberated freely in the digester and is being withdrawn in order to maintain the desired temperature and pressure conditions in the digester.

The gas and liquid so withdrawn, frequently referred to as the relief, contain valuable constituents which are recovered either by cooling or absorption and later are mixed with the cooking acids in the storage tanks for further use in the digesting process. At the completion of the cook the charge is blown by opening the blow valve and allowing the pressure in the digester to force the charge into the blow pit where the spent acid is drained off and the pulp is washed.

While the foregoing description is typical of i v this general type of digesting process, it will be understod that it is subject to great variations, depending upon the requirements of difierent mills, variations in the products, and the ideas and preferences of individual superintendents.

Due to the nature of the charge in the digester, circulation within it proceeds very slowly and the temperatures vary widely in different'parts of it. Such temperature variations necessarily produce corresponding variations in the cooking conditions, and results in the fiber in some parts of the digester becoming over-cooked and weakened before others are suitably cooked. It has been found that these conditions can be greatly improved by circulating the acid or pulp digester liquor during the cooking process, and that such circulation will reduce the time required to complete the cook and will improve both the yield and the average quality of the pulp. Because of the practical conditions under which circulation of this liquor must be produced and its highly corrosive nature, theapparatus heretofore provided for the purpose of creating and maintaining such circulation has involved a high initial expense and has proved to be expensive to maintain.

The present invention deals especially with this problem and it aims to devise a thoroughly practical solution for it.

The nature of the invention will be readily understood from the following description when read in connection with the accompanying drawings, and the novel features will be particularly pointed out in the appended claims.

In the drawings,

Figure 1 is a diagrammatic view of a digester installation embodying features of this invention;

Fig. 2 is a similar view illustrating another embodiment of the invention;

Fig. 3 is a sectional view through the upper part of a digester illustrating a different arrangement for withdrawing the liquor from the digester for recirculation;

Fig. 4 is a diagrammatic view illustrating an arrangement embodying features of this invention and designed especially for use in filling the digester with hot acid; and

Fig. 5 is a diagrammatic view illustrating another arrangement embodying: the invention.

In prior circulating systems circulation of the acid has been produced by means of pumps, in some cases installed inside the digester but more commonly located outside of it. Such systems have been made possible by the recent development of acid "resisting alloys such as those known commercially as stainless steel, Alleghany metal, and others. These installations produce definite advantages, as above indicated, in the digesting process. We have found that these same advan tages can be obtained at a reduced initial expense and lower operating costs by utilizing some of the energy of the steam fed to the digester to produce the desired circulation through the use of an inspirator. An analysis of the required rate of steam input during the digesting process compared with a consideration of the circulating requirments shows that the highest rate of steam consumption occurs during the heating and penetrating periods. Fortunately these are also the stages in which circulation of the digesting liquor is most desirable. Consequently, these conditions make the use of an inspirator entirely feasible, while at the same time producing the advantages above mentioned, avoiding the necessity for mechanical moving parts in order to create the required circulation, eliminating packing gland troubles, the use of electric motors, and the like.

Referring first to Fig. shown comprises a digester 2 which may be of any common or suitable type. Leading to this digester is a steam line 3 for conducting steam to it from a header or main 4. Frequently the flow of steam through this line is controlled or regulated automatically by a diaphragm valve, or some equivalent arrangement, and such a valve is indicated at 5. Manual control of the flow of steam is afforded by the valve 6. The relief gases and liquids withdrawn from the digester flow through the pipe 3 which runs to a cooler or acid storage tank. At the completion of the cook the charge is blown through the usual blowoif valve 10 and the pipe 9 running to the blow pit. The acid supplyline 11 through which the digester is filled with acid is connected into the steam supply line in front of the valve '7. In these respects the installation may be substantially like those commonly used heretofore.

For the purposes of the present invention a separator 12 is connected into the relief line 8 between valves 13 and 14 and is designed to effect a separation of the greater part of the liquids from the gases in the relief, the gases passing on to the cooler or storage tank and the liquids flowing down through a drop pipe 15. An inspirator 16 is connected into the steam supply line 3 and the drop pipe 15 leads into the intake leg of the inspirator. Consequentlmin such an arrangement the digester liquor discharged through the relief line will be forced back into the digester by the incoming steam, due to the action of the inspirator 16. Preferably the digester is equipped with the usual strainer, 17 to prevent the relief line from being plugged by chips. It is desirable, also, to instal a check valve 18 in the steam line behind the inspirator to prevent any backward flow of acid in the line.

Inspirators of the common types can be successfully used for this purpose'provided the parts which conduct the acids or other digester liquors are made of suitable metals to resist the action of these liquors.

If greater strainer capacity is desired than that afforded by the strainer 17, an arrangement such as that illustrated in Fig. 3 may be used. Here the relief is drawn off through a neck strainer and is conducted away by the pipe 8 but the acid is circulated through a separate pipe 8' supported by the neck of the digester and provided with a perforated or strainer section 20 which projects downwardly into the digester for a considerable distance. This pipe 8' leads directly into the drop leg 15. I

A somewhat different arrangement isillustrated in Fig. 2 in which the relief is handled in the usual manner but the acid circulating or intake pipe projects horizontally into the upper part of 1, the arrangement there the digester-and includes a perforated section 21 23 with the steam supply line at a point in front of the inspirator, this line leading to a pop valve or pressure relief valve 24. Another pop valve 25 also may be connected in the steam supply line at a point behind the inspirator, if desired.

This arrangement also makes provision for utilizing the relief from other digesters as a source of power for operating the inspirator during a part of the cook. In some systems such an arrangement can be operated mechanically. For this purpose a supply line 26 from the other digesters is led into the steam supply line 3 behind the inspirator and these two lines are equipped with valves 27 and 28, respectively, so that the inspirator can be connected with either the steam supply main 4 or with the other digesters, as desired.

In the event that it is desired to fill the digester initially with hot acid prior to starting the cook, which is a practice followed in some plants, the arrangement illustrated in Fig. 1, but modified as shown in Fig. 4, can be used. Here a supplemental steam line 30 leads from the steam main 4, through an inspirator 31 and a rotary motor driven pump 32, to the digester and is cormected to the acid supply line 11, Fig. 1. Acid from the storage supply tank is conducted to the inspirator through the pipe 33. Consequently, when the digester is being filled, steam will be admitted through the pipe line 30, and the inspirator 31 and pump 32 will cooperate with each other to force the acid into the digester, the acid being heated by mixing freely with the steam. In most installations it is necessary to use a pump in conjunction with the inspirator during the filling operation for the reason that without the assist-' ance of the pump the heat supplied to the acid would be likely to cause a separation of the gas from the acid under the low pressure conditions existing. This arrangement will be used only during filling and that illustrated in Fig. 1 will be brought into operation after the digester has been filled. The supply of acid leading to the inspirator 31 then should be cut oil and the valves 34 and 35 will be closed. Preferably an electromagnetic valve 36 is included in this steam supply main,'this valve being in circuit with the motor driving the pump 32 so that this valve will only remain open so long as the motor is in operation.

, Also, in using this arrangement care should be taken to have the inspirator located at a point where-the'incomlng acid is under sufficient presof an inspirator, lends itself readily to the. use

of an economizer or heat exchanger through which the acid can be conducted'to supply heat .to it during. the time in which it is flowing from the digester back into it again. Such an'arrangement is illustrated in Fig. 5 in which the acid is withdrawn from the digester through the pipe 21, as in the arrangement illustrated in Fig. 2, and is then forced by a motor driven pump 38 through the pipe line 39 and economizer 40 into the inspirator 16, where the steam from the steam supply line 3 picks it up and forces it back into the bottom of the digester again. The economizer or heat exchanger 40 may be heated entirely by heat that otherwise would be wasted, as for example, by 'flue gases from the boiler 41 which delivers steam to the supply line 3. In such an arrangement the pump 38 is not always necessary, but will be required if the heat transmitted to the acid in the economizer does not permit the use of steam of sufliciently high pressure to create the desired rate of circulation without producing separation of the gas from the other constituents of the acid.

It will now be appreciated that the invention provides a very simple and economical arrangement for producing the desired circulation of the acid in a sulphite digester. It not only produces the recognized advantages attending the use of the circulating step, but avoids many of the objections of former systems, while at the same time being more economical to instal, operate and maintain.

While we have herein shown and described various embodiments of our invention and the application of it to sulphite digesters, it will be understood that the invention may be embodied in other forms and may be used in other digesting processes where circulation of the digesting liquid is desirable, without departing from the spirit or scope of the invention.

Having thus described our invention, what we desire to claim as new is:

1. That improvement in methods of -maintaining circulation of the pulp digester liquor in a pulp' digester, which consists in conducting the liquid from the interior of the digester into the steam supply line leading from a boiler plant into said digester, causing part of the energy of the steam suppliedto the digester to carry said liquor into the digester, and utilizing waste heat from said boiler plant to raise the temperature of said liquor while it is-circulating outside the digester.

2. In an apparatus of the character described, the combination with a pulp digester and a steam supply line leading thereto, of an inspirator in said line, pipe connections for conducting liquor from a point in said digester to said inspirator to cause the steam flowing through said inspirator to force said liquor back into-the digester again, means for controlling the flow of steam through said line, and an automatic pressure relief valve connected with said line adjacent to said digester.

i 3. In an apparatus of the character described, the combination with a pulp digester and means for supplying steam thereto including a boiler plant and a steam supply line leading from said boiler plant to said digester, of an inspirator in said line, pipe connections for leading liquor from a point in said digester remote from the steam inlet thereto and conducting it to said inspirator to cause the steam flowing through said inspirator to force said liquor back into the digester again, and a heater through which said liquor is circulated on its way back to the digester and which is arranged to be heated by the flue gases from said boiler plant.

4. In an apparatus of the character described, the combination with a pulp digester and a main steam supply line leading thereto, of an inspirator in said line, pipe connections for conducting liquor from a point in said digester to said inspirator to cause the steam flowing through said inspirator to force said liquor back into the digester again, and means in said connections for separating the gases from the liquor withdrawn from the digester. v

5. In an apparatus of the character described, the combination with a pulp digester and a main steam supply line leading thereto, of an inspirator in said line, pipe connections for conducting liquor from a point in said digester to said in spirator to cause the steam flowing through said inspirator to force said liquor back into the digester again, and a check valve located in said steam supply line behind said inspirator to prevent any substantial flow of fluid discharged from said digester back through said steam line.

6. In an apparatus of the character described, the combination with a pulp digester and a steam supply line leading thereto, of an inspirator in said line, pipe-connections for conducting pulp digesting liquor into said inspirator to cause the steam flowing through said inspirator to force said liquor into the digester and to heat the liquor before it is so delivered to the digester,

and a pump in said connections in series with said inspirator for assisting said inspirator.

'7. In an apparatus of the character described, the combination with a pulp digester and a main steam supply line leading thereto, of an inspirator in said line, pipe connections for conducting liquor from a point in said digester to said inspirator to cause the steam flowing through said inspirator to force said liquor back into the digester again, a supplemental supply line adapt.- ed to be connected with other digesters to conduct the relief from them into said steam supply line at a point behind said inspirator, and valves in said lines for connecting said inspirator in series with either said main supply line or said supplemental line, as desired.

8. In an apparatus of the character described, the combination with a pulp digester and a main steam supply line leading thereto, of an inspirator in said line, pipe connections for conducting pulp digesting liquor into said inspirator tocausethe steam flowing through said inspirator to force said liquor into the digester and to heat the liquor before it is so delivered to the digester, a supplemental steam supply line 30, an inspirator in the latter line for forcing the initial supply of liquor into said digester and simultaneously mixing steam with said liquor to heat the liquor, a

pump in series with the latter inspirator and cooperating with it to force the liquor into said digester, and valves for enabling the operator to utilize said supply lines selectively.

9. In. an apparatus according to preceding claim 8, and arrangement in which said pump is driven by an electric motor and an electro-magnetically controlled valve 36 is included in said supplemental steam supply line and in circuit with said motor to cut off the flow of steam automatically when the motor is stopped.

' FREDERICK W. ADAMS. GEORGE S. WI'I'HAM, JR. 

